1. Field of the Invention
The present invention relates to a high-purity aqueous copper sulfonate solution and to a method of producing this solution.
2. Description of the Related Art
Copper sulfate is typically used as the copper source in copper electroplating solutions, and copper electroplating solutions based on copper sulfate and sulfuric acid are in use. Copper sulfate has a solubility concentration limit in water of approximately 80 g/L as the copper concentration, and the addition of sulfuric acid thereto results in an escalating reduction in the solubility.
Copper alkylsulfonates, e.g., copper methanesulfonate and so forth, are also used as the copper source for copper electroplating solutions. Copper methanesulfonate can be dissolved in water up to approximately 120 g/L as the copper concentration. Accordingly, changing the copper source for a copper electroplating solution from copper sulfate to copper methanesulfonate makes possible the preparation of a plating solution having a high copper concentration and enables high-speed plating film formations at high current densities; it is also effective as a ingredient salt for the execution of copper plating at high electrodeposition levels.
In recent years, copper plating in semiconductor applications such as filling through silicon vias in which plating at high current densities is possible and high electrodeposition levels are obtained is required, and the use of copper methanesulfonate has thus begun to increase. These semiconductor applications require higher purities for the plating solution and require a depletion of the metal impurities.
The following methods are known for the production of aqueous solutions of copper alkylsulfonates, e.g., copper methanesulfonate: (1) a method in which preparation is carried out by reacting copper carbonate with methanesulfonic acid (the alkylsulfonic acid) (refer to Japanese Patent Publication No. 2001-115294); (2) the electrodissolution of metal in a high-purity sulfonic acid (refer to Japanese Patent Publication No. 2006-529005). Considering these two methods further, the step in (1) in which the copper carbonate is synthesized typically uses copper sulfate and a carbonate salt, and an unsatisfactory purity on their part can result in an unsatisfactory purity for the obtained aqueous copper methanesulfonate solution; moreover, its purification/depletion has been problematic. For example, in the case of the dissolution of copper carbonate in methanesulfonic acid, sodium is present in copper carbonate at several hundred ppm and sodium is then present in the obtained aqueous copper methanesulfonate solution. Therefore, its purification/depletion has been problematic.
In the case of (2), division into an anode compartment and cathode compartment must be implemented using an anion-exchange membrane in order to prevent the copper dissolved from the anode from electrodepositing on the cathode. Anion-exchange membranes typically have the chloride ion bound as the counterion. Thus, even when metal impurities can be avoided because the metal itself is very pure, it has been difficult to prevent contamination by the chloride ion due to the use of the anion-exchange membrane. The chloride ion is used as an additive for copper electroplating solutions and is generally controlled at concentrations of several tens of milligrams per liter in a copper electroplating solution. As a consequence, when the chloride ion is present at a concentration of 10 mg/L or more in a stock solution used for the plating solution, its concentration control then becomes problematic when the plating solution is produced.
In addition, considering the effects on the plating substrate when such a solution is used as the plating solution, the pH of the aqueous copper alkylsulfonate solution is preferably relatively higher, and is preferably 2 or more when possible. The pH is generally low at less than 2 when an aqueous copper alkylsulfonate solution is prepared by method (2). The pH can also be brought to 2 or more by the use of a pH modifier; however, in this case, for example, the sodium ion, potassium ion, or ammonium ion is introduced into the solution by the pH modifier, and it results in its unavoidable admixture into the plating solution as an impurity, making this disadvantageous. The pH can also be made higher by restraining the alkylsulfonic acid concentration in order to carry out the reaction precisely in conformity to the desired copper concentration. In this case, however, the electrolyte concentration in the bath during electrodissolution ends up extremely low; the solution resistance rises and a high voltage occurs; and the durability of the anion-exchange membrane ultimately fails. According to investigations by the present inventors, the purity of the aqueous copper alkylsulfonate solution obtained by method (2) has been unsatisfactory, and in addition the initial costs have been high.